1. Field of the Invention
The present invention relates to a wireless base station and a mobile station in a mobile communication system.
2. Description of the Related Art
A mobile communication system includes plural wireless base stations, and each wireless base station forms a cell. A mobile station can perform communications by being within a cell. When the mobile station moves from one cell to another cell, a hand over process is executed between wireless base stations, so that the mobile station can continue communicating while moving from one cell to another.
FIG. 1 illustrates a frame configuration of a signal (e.g., CPICH: Common Pilot Channel, PCCPCH: Primary Common Control Physical Channel) that a mobile station receives from a wireless base station. Each frame corresponds to, e.g., 10 ms, and a system frame or a super frame is formed of 4,096 frames. The CPICH is a channel providing a reference timing in a cell, and the PCCPCH is transmitted in synchronization with the CPICH.
Next, a description is given of movements of the mobile station between cells in a standby status.
When power is turned on, the status of the mobile station becomes a standby status in a cell (serving cell) in which the mobile station is located. The mobile station periodically (intermittently) receives outgoing signals (e.g., CPICH, PICH: Page Indication Channel) from the wireless base station forming the cell. That is, in the standby state, the mobile station does not need to constantly receive signals; while the mobile station is in an energy-saving mode, the mobile station can measure levels of signals in the serving cell with CPICH and check whether a signal is sent to the mobile-station with PICH.
When the mobile station determines that it has come to an edge of the serving cell by detecting a decrease in the reception level of the CPICH of the serving cell, the mobile station executes a three-stage cell search (process of establishing frame synchronization using P-SCH, S-SCH, CPICH).
When the mobile station receives from an adjacent cell a CPICH having a sufficient reception level compared to the CPICH of the serving cell, a transfer process of standby zones is executed. Specifically, the cell to which the mobile station belongs is switched to be the adjacent cell, so that the status of the mobile station switches to a standby status for an adjacent wireless base station (also referred to as switching the cell of standby object).
When the object of standby is switched, it is necessary to acquire standby conditions in the new cell by receiving notification information (information received via the PCCPCH).
FIG. 2 illustrates a configuration of transmission information of the PCCPCH.
As shown in FIG. 2, each notification information index and each piece of notification information forms an information block corresponding to one unit of information. Two of the frames shown in FIG. 1 form one information block. For example, a block corresponding to an even number frame shown in FIG. 1 (e.g., #0) and an odd number frame shown in FIG. 1 (e.g., #1) is an information block index (#0). Encoding is performed in units of information blocks, so that one frame number is loaded in each information block. For example, in the information block corresponding to the frames #0 and #1 shown in FIG. 1, only the even frame number #0 is loaded.
Next, information sent to the mobile station is briefly described.
The notification information index indicates the type and the location of information included in the PCCPCH. For example, the notification information indices are sent in information blocks holding frame numbers that are multiples of 8, and are periodically transmitted every 80 ms.
The mobile station receives one information block of PCCPCH starting from a beginning tc of a frame that is detected by cell search, and identifies a frame number included in the information received (in this example, #4). The mobile station then determines whether the identified frame number is a multiple of eight, i.e., whether a notification information index is included. When the frame number is not a multiple of eight, the mobile station calculates a timing (tA) when the first frame (in this example, #8) having a frame number that is a multiple of eight can be received, among succeeding frames. The mobile station receives an information block again at the timing tA.
Based on the notification information index received at the timing tA, the mobile station calculates a timing (tB) when an information block holding information necessary for the standby can be received. The mobile station receives an information block at the timing tB. The information block received at the timing tB holds information about standby conditions, etc. Based on this information, the mobile station can transfer to a standby status for the adjacent wireless base station.
The mobile station synchronizes with frames (CPICH, PCCPCH) sent from the adjacent wireless base station. However, it cannot be determined whether to start receiving a frame of an even number or an odd number. Accordingly, the mobile station might start receiving a frame from a time point t′C.
However, as described above, encoding is performed in units of two frames. An information block cannot be decoded if the mobile station starts receiving a frame from the time point t′C. Thus, the mobile station receives information corresponding to 20 ms starting from t′C, and also receives information corresponding to 20 ms starting from tC, where tC is 10 ms after t′C. Among these two sets of information received, a frame number (even number) is extracted from the set of information that is decoded successfully.
Next, a description is given of a hand over operation by which a mobile station communicating in one cell moves to another cell, with reference to FIG. 3.
In response to a timeout of a timer of a predetermined time, the mobile station executes a three-stage cell search as described above to establish frame synchronization with a wireless base station that is a soft hand over destination. The mobile station receives two frames of PCCPCH from the adjacent wireless base station and acquires a frame number to be prepared for the hand over beforehand. The hand over is not performed until the reception level of CPICH of the hand over destination becomes sufficient. However, it is necessary to be prepared beforehand so that communications are not disconnected when the mobile station is moving at high speed.
The frame number is acquired to adjust the timing of receiving the same data from the hand off source and the hand off destination.
Accordingly, the mobile station calculates a timing difference (t off′) between the received frame and a frame of an outgoing data channel from the wireless base station of the soft hand over source having the same frame number as that acquired via the PCCPCH of the adjacent wireless base station. This time difference t off′ is notified to the wireless base station of the hand off source as offset information. It is assumed that the PCCPCH of the wireless base station of the soft hand over source and the outgoing data channel from the wireless base station of the soft hand over source have an offset of t off.
The wireless base station of the hand off source transfers the offset information (t off′) received from the mobile station to a wireless base station controller (RNC: Radio Network Controller), which is an upper level device. According to instructions from the RNC (according to the time difference t off′), the wireless base station of the hand off destination transmits data such that the transmission timing of an outgoing data channel to the mobile station synchronizes with the transmission timing of the data channel of the hand off source.
Accordingly, the mode of the mobile station changes to a soft hand over mode, in which the same data can be simultaneously received from the wireless base station of the hand off source and the wireless base station of the hand off destination.
As described above, when the mobile station moves to another cell (hand over) in the standby status, or when the mobile station is handed over between cells while performing communication, the mobile station needs to execute a three-stage cell search and acquire a number from a frame received from the wireless base station of the destination. Accordingly, the processing workload is increased.
In order to reduce the processing workload, there is a method of including a reference time difference RTD in the notification information. The reference time difference RTD is the time difference between a frame transmitted in the source cell and an immediate frame transmitted in the destination cell that is the adjacent cell (i.e., a time difference between arbitrary frames).
Specifically, the mobile station receives the RTD from the source cell, obtains a timing that is delayed from a frame of the source cell by the RTD, and applies the obtained timing as the frame timing of the adjacent cell. Accordingly, the three-stage cell search can be omitted. The conventional technology of using the reference time difference RTD for detecting the position of the mobile station is disclosed in, for example, Japanese Laid-Open Patent Application No. 2002-262330.
The processing workload of the cell search can be reduced by using the RTD. However, to move to another cell in a standby status, the mobile station needs to actually receive a frame transmitted from the destination cell, identify a frame holding necessary information based on a frame number included in the received data, and receive the identified frame. This processing workload cannot be reduced.
Furthermore, when a hand over is to be performed while the mobile station is communicating, the mobile station needs to actually receive a frame transmitted from the destination cell and measure the difference between a frame of the source cell having the same frame number. This processing workload cannot be reduced.